Physiological Differences Between Seasonal Dimorphs of Agonoscena pistaciae (Hemiptera: Aphalaridae) Elicit Distinct Host Plant Responses, Informing Novel Pest Management Insights

We examined differences in the physiology and life history between dimorphs of the common pistachio psyllid, Agonoscena pistaciae (Burckhardt and Lauterer) (Hemiptera: Aphalaridae), and how they differ in elicitating host plant production of key metabolites and volatile compounds involved in the recruitment of herbivores and natural enemies. Summer morphs had higher activities of glutathione S-transferase, carboxylesterase, acetylcholinesterase, and cytochrome P450 monooxygenase, superoxide dismutase, catalase, peroxidase, phenoloxidase, and a higher total protein content compared to winter morphs, whereas the latter had higher amounts of lipid, carbohydrate, and glycogen. Winter morphs were heavier, with a higher chitin content and longer preoviposition period, but greater fecundity and longevity than summer morphs. A lower LC50 to thiamethoxam for winter morphs resulted in higher mortality following exposure to the recommended rate of this insecticide in a greenhouse trial. Feeding by winter morphs elicited more strongly the release of volatile compounds known to be attractive to other herbivores, whereas feeding by summer morphs elicited more strongly the release of volatiles implicated in the attraction of natural enemies. Feeding by psyllids increased the concentrations of nitrogenous compounds, carbohydrates, vitamins, and amino acids in plants, the winter morph eliciting larger changes and more improved host plant quality. We conclude that winter morphs are more vulnerable targets for chemical control in early spring, whereas management of summer morphs could rely more on conservation biological control.

Indole primes defence signalling and increases herbivore resistance in tea plants

Upon herbivore attack, plants emit herbivore-induced plant volatiles (HIPVs). HIPVs can prime defences and resistance of intact plants. However, how HIPVs are decoded and translated into functional defence responses is not well understood, especially in long-lived woody plants. Here, we investigated the impact of the aromatic HIPV indole on defence-related early signalling, phytohormone accumulation, secondary metabolite biosynthesis and herbivore resistance in tea plants. We find that tea plants infested with tea geometrid caterpillars release indole at concentrations >450 ng*hr^-1 . Exposure to corresponding doses of synthetic indole primes the expression of early defence genes involved in calcium (Ca²⁺ ) signalling, MPK signalling and jasmonate biosynthesis. Indole exposure also primes the production of jasmonates and defence-related secondary metabolites. These changes are associated with higher herbivore resistance of indole-exposed tea plants. Chemical inhibition of Ca²⁺ and jasmonate signalling provides evidence that both are required for indole-mediated defence priming and herbivore resistance. Our systematic assessment of the impact of indole on defence signalling and deployment shows that indole acts by boosting Ca²⁺ signalling, resulting in enhanced jasmonate-dependent defence and resistance in a woody plant. Our work extends the molecular basis of HIPV-induced defence priming from annual plants to an economically important tree species.

Whitefly-induced tomato volatiles mediate host habitat location of the parasitic wasp Encarsia formosa, and enhance its efficacy as a bio-control agent

BACKGROUND

Whitefly Bemisia tabaci is a phloem-feeding insect and causes extensive agricultural damage around the world. Although the parasitic wasp Encarsia formosa is widely used to control B. tabaci on glasshouse tomatoes, low efficiency and discontinuity are frequently recorded. It has been well-documented that herbivore-induced plant volatiles (HIPVs) are important cues in the foraging behavior of the natural enemies of herbivores. However, the volatiles emitted from tomatoes infested by different developmental stages of B. tabaci (nymphs versus adults) have not been compared in terms of their effects on E. formosa attraction.

RESULTS

Olfactometer assays with four tomato cultivars revealed that the E. formosa wasps showed a significant attraction to the volatiles from adult-infested plants (except for cv. Castlemart), but not to those from nymph-infested plants. In a close-range habitat, however, the wasps appeared to use visual or tactile cues derived from nymphs for host location. Volatile analyses and behavioral assays showed that wasp attraction was correlated with enhanced β-myrcene and β-caryophyllene emissions from adult-infested plants. Furthermore, the use of B. tabaci adult-induced plant cues under glasshouse conditions resulted in a higher parasitism rate by this parasitoid.

CONCLUSION

Our findings confirm that E. formosa uses the HIPVs resulting from feeding of B. tabaci adults to locate host habitat. Release of β-myrcene and β-caryophyllene from dispensers may enhance the efficacy of E. formosa as a biological control agent against B. tabaci in glasshouse production systems.

Attracting Chrysopidae With Plant Volatiles for Lace Bug (Hemiptera: Tingidae) Control in Rhododendrons and Azaleas

The azalea lace bug (Stephanitis pyrioides Scott) (Hemiptera: Tingidae) is an invasive pest of rhododendrons and azaleas (Ericaceae: Rhododendron), which feeds on the underside of leaves causing chlorosis, reduced photosynthesis, and even plant death. While insecticides can control this pest, growers, landscape managers, and homeowners have requested softer alternatives. Augmentative release of predatory green lacewing Chrysoperla sp. (Neuroptera: Chrysopidae) eggs and larvae has reduced S. pyrioides, but large-scale implementation may not be practical nor cost-effective. Attracting naturally occurring Chrysopidae with plant volatiles may be an economical and convenient option. In this study, we tested whether volatile blends 1) attracted Chrysoperla sp., and 2) controlled S. pyrioides populations on Rhododendron spp. in farm or urban landscapes. Experimental plots contained different multicomponent lures placed aboveground next to infested plants. Adult Chrysoperla sp., other natural enemies, and S. pyrioides from egg to adult stages were monitored in both farm and urban landscapes for two summers. Overall, two out of three volatile blends consistently attracted Chrysoperla sp. to sticky traps near baited plants. Methyl salicylate + acetic acid + 2-phenylethanol (methyl salicylate blend) and acetophenone + acetic acid + 2-phenylethanol (acetophenone blend) captured more adult Chrysoperla sp. than control traps in farm landscapes. However, only the acetophenone blend was associated with a slight reduction of S. pyrioides. Additional research is needed to determine whether the phenology of the first generation of both species are synchronized for effective season biological control in the Pacific Northwest.

Trade-off between defense priming by herbivore-induced plant volatiles and constitutive defense in tomato

BACKGROUND

Herbivore-induced plant volatiles (HIPVs) can prime plant defenses and enhance herbivore resistance in neighboring plants. Although a trade-off between constitutive defense and HIPV emission has been documented in many plant species, little is known about the effect of HIPV variation on defense priming, and whether there is a trade-off between defense priming and constitutive defense in neighboring plants.

RESULTS

Using three tomato genotypes, including two wild types [Moneymaker (MM) and Castlemart (CM)] and one jasmonic acid (JA) overexpression 35S::prosys genotype, we investigated the effects of exposure to volatiles from plants infested by beet armyworm (Spodoptera exigua) caterpillars on the defenses and resistance of conspecific neighboring plants. We also analyzed the HIPV emissions from the three genotypes and their constitutive defense and resistance. Exposure to volatiles from S. exigua-infested MM plants primed an array of defensive responses (including the accumulation of JA and PI-II protein and the expression of the defense genes PI-I and PI-II), and enhanced plant resistance against the caterpillars. In contrast, exposure to volatiles from S. exigua-infested CM or 35S::prosys did not result in defense priming or an increase in plant resistance. Analyses of HIPVs and gene expression indicated that defense priming in MM was due to enhanced emission of β-ocimene and linalool. We further demonstrated that levels of constitutive defense and resistance are low in MM and high in CM and 35S::prosys, suggesting a negative correlation between defense priming and constitutive defense.

CONCLUSION

Our findings suggest that there is a trade-off between defense priming by HIPVs and constitutive defense in tomato. This suggests that defense priming should be used with caution in agriculture. © 2020 Society of Chemical Industry.

Innate and Learned Responses of the Tephritid Parasitoid Psyttalia concolor (Hymenoptera: Braconidae) to Olive Volatiles Induced by Bactrocera oleae (Diptera: Tephritidae) Infestation

Parasitic wasps can learn cues that alter their behavioral responses and increase their fitness, such as those that improve host location efficiency. Psyttalia concolor (Szépligeti) (Hymenoptera: Braconidae) is a koinobiont endoparasitoid of 14 economically important tephritid species, including the olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae). In this research, we investigated the nature of olfactory cues mediating this tritrophic interaction. First, we identified the chemical stimuli emanating from uninfested and B. oleae-infested olive fruits via solid phase microextraction and gas chromatography-mass spectrometry analyses and identified >70 volatile organic compounds (VOCs). Two of these were increased by B. oleae infestation, (E)-β-ocimene and 2-methyl-6-methylene-1,7-octadien-3-one, and four were decreased, α-pinene, β-pine ne, limonene, and β-elemene. Innate positive chemotaxis of mated P. concolor females toward these VOCs was then tested in olfactometer assays. Females were attracted only by (E)-β-ocimene, at both tested dosages, indicating an intrinsic response to this compound as a short-range attractant. Next, we tested whether mated P. concolor females could learn to respond to innately unattractive VOCs if they were first presented with a food reward. Two nonassociative controls were conducted, i.e., "odor only" and "reward only." Following training, females showed positive chemotaxis toward these VOCs in all tested combinations, with the exception of limonene, a VOC commonly produced by flowers. Control females showed no significant preferences, indicating that positive associative learning had occurred. These results clarify how learned cues can fine-tune innate responses to B. oleae-induced VOCs in this generalist parasitoid of tephritid flies.

Electroantennogram and behavioral responses of Cotesia plutellae to plant volatiles

Plant volatiles have been demonstrated to play an important role in regulating the behavior of Cotesia plutellae, a major larval parasitoid of the diamondback moth (DBM), Plutella xylostella, but little is currently known about the function of each volatile and their mixtures. We selected 13 volatiles of the DBM host plant, a cruciferous vegetable, to study the electroantennogram (EAG) and behavioral responses of C. plutellae. EAG responses to each of the compounds generally increased with concentration. Strong EAG responses were to 100 μL/mL of trans-2-hexenal, benzaldehyde, nonanal and cis-3-hexenol, and 10 μL/mL of trans-2-hexenal and benzaldehyde with the strongest response provoked by trans-2-hexenal at 100 μL/mL. In the Y-tube olfactometer, C. plutellae, was significantly attracted by 1 μL/mL of trans-2-hexenal and benzaldehyde. β-caryophyllene, cis-3-hexenol or trans-2-hexenal significantly attracted C. plutellae at 10 μL/mL, while nonanal, benzyl alcohol, cis-3-hexenol or benzyl cyanide at 100 μL/mL significantly attracted C. plutellae. Trans-2-hexenal significantly repelled C. plutellae at 100 μL/mL. EAG of C. plutellae showed strong responses to all mixtures made of five various compounds with mixtures 3 (trans-2-hexenal, benzaldehyde, nonanal, cis-3-hexenol, benzyl cyanide, farnesene, eucalyptol) and 4 (trans-2-hexenal, benzaldehyde, benzyl alcohol, (R)-(+)-limonene, β-ionone, farnesene, eucalyptol) significantly attracting C. plutellae. These findings demonstrate that the behavior of C. plutellae can be affected either by individual compounds or mixtures of plant volatiles, suggesting a potential of using plant volatiles to improve the efficiency of this parasitoid for biocontrol of P. xylostella.

Maize plants prime anti-herbivore responses by the memorizing and recalling of airborne information in their genome

Intact maize plants prime for defensive action against herbivory in response to herbivore-induced plant volatiles (HI PVs) emitted from caterpillar-infested conspecific plants. The recent research showed that the primed defense in receiver plants that had been exposed to HI PVs was maintained for at least 5 d after exposure. Herbivory triggered the receiver plants to enhance the expression of a defense gene for trypsin inhibitor (TI ). At the upstream sequence of a TI gene, non-methylated cytosine residues were observed in the genome of HI PV-exposed plants more frequently than in that of healthy plant volatile-exposed plants. These findings provide an innovative mechanism for the memory of HI PV-mediated habituation for plant defense. This mechanism and further innovations for priming of defenses via plant communications will contribute to the development of plant volatile-based pest management methods in agriculture and horticulture.

The herbivore-induced plant volatile methyl salicylate negatively affects attraction of the parasitoid Diadegma semiclausum

The indirect defense mechanisms of plants comprise the production of herbivore-induced plant volatiles that can attract natural enemies of plant attackers. One of the often emitted compounds after herbivory is methyl salicylate (MeSA). Here, we studied the importance of this caterpillar-induced compound in the attraction of the parasitoid wasp Diadegma semiclausum by using a mutant Arabidopsis line. Pieris rapae infested AtBSMT1-KO mutant Arabidopsis plants, compromised in the biosynthesis of MeSA, were more attractive to parasitoids than infested wild-type plants. This suggests that the presence of MeSA has negative effects on parasitoid host-finding behavior when exposed to wild-type production of herbivore-induced Arabidopsis volatiles. Furthermore, in line with this, we recorded a positive correlation between MeSA dose and repellence of D. semiclausum when supplementing the headspace of caterpillar-infested AtBSMT1-KO plants with synthetic MeSA.